Make Coil Spring?

[Crunch]

Hello, all,

I'm sure this has been discussed at length, but searching doesn't find much. I need a compression coil spring about 1/2" OD x 1-3/4" long which compresses closed with a load of about 35-50# for a chainsaw pull-start shock absorber. (I know they sell them but I want to make one with a spring.) 

Was thinking of using the steel from a larger-diameter spring for spring stock, heating until I could form it around a suitable sized rod mandrel, then quench and retemper.

Is there a down and dirty way of doing this, especially the retemper? I've heard of using molten lead as a temp regulator but would like something easier. Does a kitchen oven get hot enough to retemper your typical small coil spring?

Thanks in advance for any help.

[Tubalcain2]

Yeah, a kitchen oven will definitely be hot enough. I would guess you would want a dark straw color. Someone will probably come along and correct me on that any minute. 

[Steve Sells]

what is this retemper you are talking about?  Please, using standard terms is the best way to get a meaningful answer

try reading the pinned posts on heat treating

[JNewman]

I would not try to shop heat treat a a spring in a situation where a failure is likely to cause injury.

However in your case what I would do is just air quench the spring.  A kitchen oven is not even close to being hot enough. The size of the spring you are talking about will cool quickly enough to harden in air.  The residual heat will then temper it. You will not get as good a result as a commercialy heat treated spring.  But it should be acceptable.  If it turns out to not be a hard enough at least you don't have to start from scratch, you just need to heat treat again. If it turns out not to be hard enough the next time I would try using a fan to cool it until the colour disappears and then let it air cool.

When heat treating it i would use a small wire to hold the spring an hang it from until it is cool.  Tongs or anything cooler you set the spring on will cool the spring too quickly and unevenly

[BIGGUNDOCTOR]

You can buy spring temper wire and then you just wind it around a mandrel cold. A lathe really helps with that part. Personally I would just go to any supplier like McMaster Carr and just buy a spring with the specs needed. Far less hassle, and far less expensive.

[Frosty]

You might look at a small engine valve spring.

You could learn to pull the rope without pulling it to the stop. Or maybe get your saw tuned up.  When properly tuned and the gas isn't too oil rich my Stihls will start on one pip from the piston so I don't have to pull it more than maybe 10"-12" say about half way to the end of the pull rope.

Frosty The Lucky.

[Crunch]

Thanks for the replies, guys.

Steve, sorry for the terrible terminology. I guess I should have said "temper" rather than retemper, but since the spring was (I assume) quenched and tempered once already, I was calling it a "retemper" but I guess since by then I would have quenched it again (re-quenched?), it would be a "temper" – not a "retemper." Again, my apologies.  

JNewman, thanks, air hardening is probably the best bet. Thanks for the tips.

Biggundoctor, I didn't know about that spring temper wire. Unfortunately, no good suppliers around here, and shipping from online (if I could even find what I want online, and I can't) would be prohibitively expensive.

Frosty, thanks. Hadn't thought of the valve spring. I'll look around in my junk pile. FWIW, it's not an issue of "pulling the rope all the way out to the stop" – the issue is, it's a Stihl 660 with a 7hp motor, and sometimes it pops and wants to rip your hand off...which I guess is why Stihl makes "Elastostart" handles that absorb some of that shock...but I'd rather make something than buy it...

Thanks again.